1. Field of the Invention
The present invention relates to the process of semiconductor manufacturing. More specifically, the present invention relates to a method and an apparatus for accurately modeling an arbitrarily polarized illumination source in an optical lithography system used in a semiconductor manufacturing process.
2. Related Art
Dramatic improvements in semiconductor integration circuit (IC) technology presently make it possible to integrate hundreds of millions of transistors onto a single semiconductor IC chip. These improvements in integration densities have largely been achieved through corresponding improvements in semiconductor manufacturing technologies. Semiconductor manufacturing technologies typically include a number of processes which involve complex physical and chemical interactions. Since it is almost impossible to find exact formulae to predict the behavior of these complex interactions, developers typically use process models which are fit to empirical data to predict the behavior of these processes. In particular, various process models have been integrated into Optical Proximity Correction (OPC)/Resolution Enhancement Technologies (RET) for enhancing imaging resolutions during optical lithographic processes.
More specifically, during an OPC/RET modeling process, one or more process models are used to make corrections to a semiconductor chip layout in a mask to compensate for undesirable effects of complex lithographic processes. An OPC/RET/DFM(Design for manufacturing) model (“OPC model” hereafter) is typically composed of a physical optical model and an empirical process model. An OPC simulation engine uses the OPC model to iteratively evaluate and modify edge segments in the mask layout. In doing so, the OPC simulation engine computes the correct mask patterns which produce physical patterns on wafers that closely match a desired design layout. Note that the effectiveness of the corrected mask patterns is typically limited by the accuracy of the OPC model.
As Moore's law drives IC features to increasingly smaller dimensions (which are now in the deep submicron regime), a number of physical effects, which have been largely ignored or oversimplified in existing OPC models, are becoming increasingly important for OPC model accuracy. Hence, it is desirable to provide more comprehensive, physics-centric descriptions for these physical effects to improve OPC model accuracy.
In particular, the polarization behavior of an optical lithographic system is one of the physical effects that are inadequately represented in a traditional OPC model. While existing OPC models can model the polarization behavior of light and optical lithographic systems in some very limited aspects (i.e., polarization-state-dependent refraction, transmission and reflection in thin films on a wafer), these models are not capable of modeling the more complex polarization-state-dependent vectorial behavior of light in an illumination source and in a projection lens pupil of the lithographic system.
More specifically, the existing OPC models treat a projection lens system as a simple scalar lens pupil, which acts on the incoming optical field homogeneously and independently of the polarization state of the optical field. Consequently, these models cannot accurately and adequately capture the polarization state change of the incident field imposed by the projection lens system. Furthermore, the existing OPC models treat the whole illumination source as either an unpolarized light or a single state (TE/TM/X/Y) uniformly polarized light, while a realistic illumination source can have a much more complex polarization state which varies with location within the illumination source pupil. These oversimplified illumination source models make the modeling accuracy and fidelity of an OPC model inadequate for ever-decreasing feature sizes.
Hence, what is needed is a method and an apparatus that can accurately model an arbitrarily polarized illumination source without the above-described problems.